Ultrasonic diagnostic apparatuses have been widely used because of their ability to non-invasively obtain tomographic images of a subject such as a living body. In recent years, there have been apparatuses with a two-dimensional array of piezoelectric elements and apparatuses that obtain high-resolution images by using technology called “harmonic imaging.” Harmonic imaging generates an image that shows an internal state of a subject, by using, not a frequency (fundamental wave) component of an ultrasonic wave transmitted from an ultrasonic probe into a subject, but a harmonic component, which is extracted from strain components that are generated by the nonlinearlity of the subject when an ultrasonic wave propagates through the subject. As a piezoelectric element that is suitably used in this harmonic imaging, Patent Document 1 proposes an ultrasonic diagnostic apparatus in which a thin organic piezoelectric layer (polyvinylidine difluoride; PVDF) capable of receiving a high-frequency signal of the harmonic component with a high degree of sensitivity is laminated on an inorganic (ceramic) piezoelectric layer (lead zirconium titanate; PZT) capable of high-power transmission. This type of organic-inorganic laminated structure or a structure with a two-dimensional array of piezoelectric elements has an enormous number of signal lines connected to the respective piezoelectric elements. Thus, the layout of the signal lines needs to be devised.
FIG. 14 shows how conventional technology is used for laying out the signal lines in a two-dimensional array of piezoelectric elements. After laminating an individual electrode 2 and a piezoelectric layer 3 on a backing layer 1, the piezoelectric elements are separated into pieces (element isolation). A filler 4 fills the gaps between the piezoelectric elements to flatten the obtained laminated structure, and then a common (GND) electrode 5 and a plurality of acoustic matching layers 6, 7 are laminated thereon. Individual signal lines 8 that are connected respectively to the obtained individual electrodes 2 pass through the backing layer 1 so as to be connected to a wiring substrate, not shown, which is disposed below the backing layer 1.
The backing layer 1 generally attenuates the vibration of the piezoelectric layer 3 that is generated after transmission of the ultrasonic wave, and absorbs the transmitted ultrasonic wave that is emitted rearward, in order to minimize the disturbance of the transmitted ultrasonic wave, which is caused by light reflection, as well as to reduce the pulse width of the transmitted ultrasonic wave. For this reason, the backing layer 1 needs to have a certain thickness. Reducing the pitch of the arrayed piezoelectric elements increases the aspect ratio of through-holes that allow the passage of the signal lines 8, complicating the processing operation. Moreover, finely arranging the elements two-dimensionally increases the output impedance of the piezoelectric elements, causing great transmission loss due to the wiring capacity.
In order to deal with such problems, in Patent Document 2, a thin integrated circuit that is made translucent to ultrasonic waves is interposed between a backing layer and a piezoelectric layer, so that the number of signal lines can be reduced. In addition, the signal lines are pulled out to the side of the array.
Patent Document 3, on the other hand, discloses the use of silicon as the acoustic matching layers.
However, in the technology disclosed in Patent Document 2, an additional integrated circuit board needs to be provided between the backing layer and the piezoelectric layer.
The technology disclosed in Patent Document 3, on the other hand, simply adopts silicon as the material of the acoustic matching layers and uses a side surface of the backing layer to lay out signal lines to be connected to the piezoelectric elements. Therefore, it is difficult to apply the technology disclosed in Patent Document 3 to a device that has, for example, a two-dimensional array of piezoelectric elements or a device that has a laminated structure formed by an organic piezoelectric element and inorganic piezoelectric element.    Patent Document 1: Japanese Patent Application Publication No. 2004-208918    Patent Document 2: Japanese Translation of PCT Application No. 2007-513563    Patent Document 3: Japanese Patent Application Publication No. 2003-125494